Personally, I like 3rd person much better. It enables you to be more objective with your description, which might be important in a sci-fi book. That way you can explain some of the technology or background without your main character having to know (or find out somehow) how things work out or what happened.

I struggle to write in the 1st person because the way your character sees the world really needs to color your story if you're going to present it from that viewpoint and I think that's where a lot of YA books fall short. They spend so much time showing us how the main character sees things that we don't get a big enough picture of what their world really is. This isn't a problem if it's a modern day novel, but i don't know that it would work out so well for sci-fi. I'm writing a YA book as well but it's fantasy and i plan on using the 3rd person.

Hope that helps a little bit. I'd love to hear more about your story.

~Athalia